“And the Twelve were with him, as were certain women who had been cured of wicked spirits and sicknesses: Mary who was called Magdalene, from whom seven demons had come out […].”
~Luke 8:1,2

“As Jesus got out onto land, a demon-possessed man from the city met him. For a considerable time he had not worn clothing, and he was staying, not in a house, but among the tombs.”
~Luke 8:27-36

How does a person succumb to demon possession and why did it seem to be so commonplace in Jesus’ day?
“Insight On The Scriptures” defines it as “the captive control and influence of a person by an invisible wicked spirit.”
That influence may manifest itself physically, emotionally or mentally through the victim.
Luke’s account demonstrates that it is possible for a person to be possessed by more than one demon, and it is possible for a demon to possess an animal.
When a person opens a portal into the occult, be it by superstitious practices, trying to communicate with the dead, diabolic entertainment, or through witchcraft, he makes himself vulnerable to demon possession.
In the case of King Saul, a demon began to attack him when he became arrogant and defiant in his service to Jehovah and lost God’s holy spirit. (1 Sam. 15:10,11,22,23; 16:14-16,23; 18:10-12)
When Jesus walked the earth, many Jewish leaders in particular had strayed from true worship and their attitude was heavily influenced by demons. (John 8:44)
Modern Christians do not practice exorcism, which tends to combine rites and chants or a combination of words, as if a person could be liberated from demons through magic.
Rather, they put on “the complete suit of armor” of Christian lifestyle and outright reject evil practices. (Eph. 6:11-18; Jas. 4:7)